1. Field of the Invention
Currently practiced processes for beneficiation of phosphate ores commonly utilize flotation as the principal step for separating phosphates from gangue minerals. The "Crago" or "double float" froth flotation process is commercially used for beneficiating such phosphate ores in which silica and silicate minerals are the prodominant constituents of gangues. That process consists of conditioning the material with fatty acids and fuel oil, flotation of phosphate minerals, deoiling with sulfuric acid to remove the reagents, and refloating with amine reagents to remove the siliceous gangue which either floated or was trapped in the rougher float (U.S. Pat. No. 2,293,640).
However, many phosphate ores of potential commercial value contain carbonate gangue mineral matter in addition to siliceous minerals. These carbonate mineral impurities include dolomite, calcite, dolomitic limestone, seashell, and other less common carbonate-type minerals. Such mineral impurities not only dilute the P.sub.2 O.sub.5 content of the phosphate ore concentrate, but also may interfere in subsequent chemical processing thereof. For example, carbonate minerals present in phosphate ore concentrates used to produce phosphoric acid, superphosphate, or triple superphosphate consume acid reagents in the acidulation steps without providing additional fertilizer values. They aggravate foam formation in the reactor vessel. The presence of dolomite or dolomitic limestone in the phosphate ore concentrate is particularly important because relatively small amounts of magnesium (i.e., &gt;1 percent MgO) may cause technical problems in current wet-process acid chemical plants due to increased viscosity of acid, increased defoamer usage, scale and sludge formation, and possibly difficulty in maintaining a standard diammonium phosphate (DAP) fertilizer grade.
The "double float" process has generally been ineffective for beneficiating such high carbonate phosphate ores. Collectors such as fatty acids used for phosphate minerals generally float carbonate minerals as well, and it is only by the employment of suitable selective depressants of either phosphate or carbonate gangue that adequate selectivity can be obtained.
2. Description of the Prior Art
Methods of beneficiating phosphate ores containing carbonate and siliceous gangue materials are described in the following U.S. Pat. Nos.: 3,259,242, Snow, July 1966; 3,462,016, Bushell et at, August 1969; 3,462,017, Bushell et al, August 1969; 3,807,556, Johnston, May 1974; 4,144,969, Snow, March 1979; 4,189,103, Lawver et al, February 1980; 4,287,053, Lehr et al, September 1981; 4,317,715, Kintikka et al, March 1982; 4,364,824, Snow, Dec. 21, 1982; 4,372,843, Lawver et al, February 1983; 4,636,303, Hsieh et al, Jan. 13, 1987; and 4,648,996, Hsieh et al, Mar. 10, 1987. Each of these patents deals with processes that differ from the present invention, as for example: Snow, in U.S. Pat. No. 3,259,242 supra describes the beneficiation of a macro-crystalline apatite-calcite mixture which utilizes a cationic flotation to collect apatite as a float concentrate. Bushell et al, in U.S. Pat. No. 3,462,016 supra, teaches the use of H.sub.3 PO.sub.4 and Bushell et al in U.S. Pat. No. 3,462,017 supra teach the use of NH.sub.4 H.sub. 2 PO.sub.4, both as phosphate mineral depressants for beneficiating carbonate-bearing phosphate rock. Johnston, in U.S. Pat. No, 3,807,556 supra further teaches the use of soluble sulfate salts as Na.sub.2 SO.sub.4 or (NH.sub.4).sub.2 SO.sub.4 to reduce the loss of soluble phosphate in the processes of U.S. Pat. Nos. 3,462,016 and 3,462,107 supra.
The use of an apatite-collecting cationic reagent and a liquid hydrocarbon is described in U.S. Pat. Nos. 4,144,969 and 4,189,103 supra. These patents describe a phosphate ore beneficiating process in which the deslimed ore is first subjected to a "double float" froth flotation as described in U.S. Pat. No. 2,293,640, Crago, Aug. 18, 1942, to remove siliceous gangue. The float product containing apatite with dolomite impurities is then conditioned with a cationic agent and a liquid hydrocarbon to concentrate apatite as float.
Hintikka et al, in U.S. Pat. No. 4,317,715 supra, describe two alternative processes to separate phosphate minerals from carbonate minerals after phosphate and carbonate minerals are concentrated from silicates; (1) the combined phosphate-carbonate concentrate is treated with a base, whereafter phosphate mineral is floated with a cationic collector, or (2) the combined phosphate-carbonate concentrate is treated with SO2 or CO2 to recover phosphate as a float.
Snow in U.S. Pat. No. 4,364,824 and Lawver et al in U.S. Pat. No. 4,364,824 supra, describe a flotation process for removing carbonate mineral impurities. The process employs salts of sulfonated linear fatty acids as a carbonate mineral collector and a phosphate depressant, such as polyphosphate, metaphosphate, pyrophosphate or orthophosphate, to remove carbonate minerals in the overflow and to collect the phosphate concentrate in the underflow.
Lehr et al in U.S. Pat. No. 4,287,053, assigned to the assignee of the present invention, describe a phosphate ore beneficiating process in which phosphate ores containing carbonate mineral impurities are subjected to froth flotation in the presence of phosphate depressants, said depressants comprising alkyl diphosphonic acids, and a carbonate collector, said collector comprising fatty acids, removing the separated carbonate minerals from the overflow, and recovering the phosphate value in the underflow.
Hsieh in U.S. Pat. No. 4,636,303, assigned to the assignee of the present invention, teaches a phosphate ore beneficiating process wherein phosphate ore containing coarse, weathered dolomite in the flotation feed thereof is subjected to a rougher phosphate flotation with fatty acid and fuel oil and wherefrom is removed the coarse, weathered dolomite in one or more cleaner phosphate flotations, without the use of additional flotation reagents added thereto. After the deoiling or acid-wash step, the resulting concentrate is subjected to amine flotation to remove residual silica and/or to carbonate flotation using alkyl diphosphonic acid as depressant and fatty acids as collector, with the phosphate values being recovered in the underflow.
Hsieh in U.S. Pat. No. 4,648,966, assigned to the assignee of the present invention, teaches still further an improved process for beneficiating phosphate ore containing coarse, weathered dolomite in the flotation feed. The teachings thereof introduce the use of two additional steps, i.e., (1) subjecting the float from cleaner phosphate flotation to a sulfuric acid-based partial deoiling step, and (2) subjecting said partial deoiling pulp to a phosphate-carbonate flotation step to collect the first fraction of the phosphate concentrate as underflow. The float is then subjected to total deoiling with sulfuric acid and then to carbonate flotation step using diphosphonic acid as a phosphate mineral depressant and fatty acid as a carbonate mineral collector to recover the remaining phosphate values as underflow.
The present invention teaches a process for beneficiation of dolomitic phosphate ores which is much simpler than those of the prior art. In the practice of the instant invention, the prepared flotation feed is conditioned with fatty acid and fuel oil which is utilized therein as the phosphate mineral collector and then conditioned with humic acid which is utilized therein as the dolomite depressant or modifier. The conditioned flotation feed is then subjected to rougher phosphate flotation to reject both the dolomite and the silica in the underflow or sink and to collect the rougher phosphate concentrate in the overflow or float. The rougher phosphate concentrate is then subjected to one or more cleaner phosphate flotation steps, passes, or stages, without addition thereto of additional flotation reagent, to remove additional silica and dolomite in the underflow. The phosphate product is then recovered as cleaner float. In practice of this instant invention, we have found that a sufficient amount of dolomite impurity is removed in the rougher and cleaner phosphate flotation steps such that no carbonate flotation is required to remove residual dolomite particles as is required in the practice of the teachings in U.S. Pat. Nos. 4,636,303 and 4,648,966 supra.